This invention relates to the manufacture of C.sub.2 and C.sub.3 olefins from gas oil in a higher yield than was previously attained. In particular, it relates to manufacturing olefins from gas oil feedstocks that contain low concentrations of benzothiophenes, either because the feedstock has been selected for low benzothiophenes concentration or because benzothiophenes have been removed from the feedstock.
Gas oil is a crude oil distillation cut primarily consisting of a mixture of C.sub.6 to C.sub.16 hydrocarbons. Olefins can be manufactured from gas oil by cracking these hydrocarbons into smaller units. Cracking is accomplished by pyrolysis--heating the gas oil at high temperatures in the presence of steam and the absence of oxygen. Cracking produces a mixture of olefins of different chain lengths. Ethylene and propylene are the olefins which have the greatest commercial value.